Composition and use of the composition for the extraction of metals from aqueous solution

ABSTRACT

A composition for use in extracting copper values from aqeous solutions of metal salts which comprises one more o-hydroxyaryloximes containing at least 5 aliphatic or alicyclic carbon atoms which are strong metal extractants which, in 0.2 molar solution in an aliphatic hydrocarbon solution loaded with 50% of the theoreticl uptake of copper, will be in equilibrium with 0.1 molar solution of copper as copper perchlorate at a pH less than 1; and one or more branched chain aliphatic or aromatic-aliphatic alcohols containing 14 to 30 carbon atoms or aliphatic or aliphatic-aromatic esters containing 10 to 30 carbon atoms, wherein the ratio of the number of methyl carbon atoms to the number of non-methyl carbon atoms is higher than 1:5, and which is selected from the group consisting of highly branched isohexadecyl alcohol, highly branched isooctadecyl alcohol and a diester of 2,2,4-trimethyl-1,3-pentanediol; and wherein the weight ratio of A to B is in the range 10:1 to 1:3.

This application is a continuation-in-part of U.S. application Ser. No.06/863,031, filed May 14, 1986, now abandoned.

This invention relates to an improvement in compositions and processesfor extracting metals from aqueous solutions, especially solutionsobtained by leaching ores with acids, using o-hydroxyaryloximes asextracting agents.

It is known to extract metals, especially copper, from aqueous solutionscontaining the metal in form of, for example, a sulphate salt bycontacting the aqueous solution with a solution of an o-hydroxyaryloximein a water immiscible organic solvent and then separating the solventphase loaded with metal, i.e. containing a part of the metal in the formof a chelate compound with the o-hydroxyaryloxime. The metal can then berecovered from the metal loaded solvent phase by stripping with acidsolutions followed, for example, by electrowinning.

The reaction leading to the metal chelate compound also forms acid andcauses a lowering of the pH. This reaction is reversible and proceeds toan equilibrium point which will favour formation of the chelate compoundas the pH is increased. The metal salt-containing aqueous solutions fromwhich metal e.g. copper is to be extracted will frequently be leachliquors obtained by extracting metal ores with acid and will in somecases have a low pH. Since the amount of chelate compound formed atequilibrium is lower as the pH is decreased only thoseo-hYdroxyaryloximes which have a strong chelating power will be able toachieve a high degree of extraction from those aqueous leach liquorshaving very low pH or high copper content.

The advantage of high copper extraction shown by these stronglychelating oximes is to some extent offset by the large amount of copperwhich remains as chelate in the solvent after stripping with acid ofconvenient strength. While this residual copper as chelate is not lostsince it can be recycled to the extraction stage, a reduction in theamount of residual copper chelate would, in the absence of anycomparable reduction in the degree of copper extraction from the aqueoussolution, afford an improvement in the overall efficiency of theprocess.

In our UK Patent No 1549615 we show that the amount of copper removed inthese oases from the solvent phase in the stripping stage issignificantly increased if the solvent phase contains a defined phenol.Such compounds are sometimes called `strip modifiers`.

In the above specification we also disclose that certain aliphaticalcohols, such as tridecanol have similar beneficial effects.

Modifiers will not only influence the strength of the extractant, butcan also affect the hydrolytic stability, the selectivity of copperextraction over iron extraction, the level of entrainment, the kineticsof the extraction and stripping stages and the generation of crud. Asuitable modifier will therefore often be the result of a compromise.

`Crud` is a term applied to undesirable extraneous matter formed at theorganic-aqueous interface or in the organic phase in the settlercompartment of mixer settlers used in the solvent extraction process. Itis usually an oil-water emulsion stabilised by the presence of finelydivided solid material that may be either alumino silicates present inthe feed, or colloidal silica precipitated during the solvent extractionoperation. It can accumulate in sufficient quantities to seriouslyreduce the working volume of a settler leading to flooding. Where largequantities are produced it has to be removed and the emulsion broken bycentrifuging. Crud can also be a source of loss of reagent by aborptionon the silicacious solids component which is discarded.

In solvent extraction operations employing banks of mixer settlersoperated in a continuous fashion, it is inevitable that after theprimary separation of the organic and aqueous phases in the settlers,there remains some entrainment of one phase in the other. This is in theform of minute droplets that are very slow to coalesce or settle and arethus carried through with the primary phase. In the case of droplets oforganic material entrained in the aqueous phase this represent a majorloss of extractant reagent from the plant, both in organic materialentrained in the discarded raffinate from the extraction circuit and inorganic material transferred to the electrolyte in the stripping stage.In the latter case the entrained organic material may cause furthercomplications by interfering with the clean deposition of copper and maycause burning of the elctrodes. In the case of entrainment of dropletsof aqueous in organic phase, this represents a means of physicaltransfer of unwanted metals such as iron present in the aqueous feedsolution, that may outweigh the advantages of the high selectivity of anextractant reagent for copper over other metals. It is therefore seen tobe of considerable advantage to maintain entrainment at as low a levelas possible. Various physical means have been tried but it is alsoclearly a function of the reagent composition employed and there aredistinct advantages in reagents which minimise formation of entrainmentof one phase in another.

U.S. patent specification No. 4,507,268 and 4,544,532 describes the useof mixture of aldoxime and ketoxime with no or only low level ofmodifiers. Such mixtures will be advantageous in respect of crudformation. These formulations commercially known under the trade namesLix 864 (2 hydroxy 5 dodecyl benzaldoxime with 2 hydroxy 5 nonylbenzophenone oxime) and Lix 984 ( 2 hydroxy 5 dodecyl benzaldoxime with2 hydroxy 5 nonyl acetophenone oxime) will however provide extractionwith low Cu over Fe selectivity.

There is thus still a need for more efficient modifiers with goodselectivity which furthermore will not favour formation of crud andavoid entrainment.

We have now found that the use of highly branched chain aliphatic oraliphatic-aromatic C-10 C-30 esters or C-14-C30 alcohols give unexpectedbenefits as strip modifiers. Good and unexpected selectivity for copperover iron can be achieved and the above disadvantage concerning crudformation and entrainment level can be overcome by using such compounds,particularly very highly branched derivatives.

It has also been found quite unexpectedly that the formulations inaccordance with the present invention provide better hydrolyticstability than formulations based on certain mixture of aldoxime andketoxime without modifier as claimed in U.S. Pat. No. 4,507,268.

Accordingly, our invention provides a composition for use in extractingmetals from aqueous solutions of metal salts which comprises :

A. on or more o-hydroxyaryloximes containing at least 5 aliphatic oralicyclic carbon atoms which are strong metal extractants as hereinafterdefined.

B. one or more branched chain aliphatic or aromatic-aliphatic alcoholscontaining 14 to 30 carbon atoms or esters containing 10 to 30 carbonatoms, the ratio of methyl carbon atoms to non-methyl carbon atoms beinghigher than 1 : 5, the weight ratio of A to B being in the range 10:1 to1:3.

Preferably the methyl carbon atoms to non methyl carbon atoms ratio ishigher than 1 : 3 and the weight ratio of A to B is 5:1 to 1:1. Theesters contain preferably 14 to 25 carbon atoms and the alcohols 15 to25 carbon atoms.

The composition may be dissolved in an organic solvent, which for thenormal metal extraction process should be water immiscible.

A further aspect of the present invention is a process for extractingmetal from aqueous solution by the steps of:

a. contacting the aqueous solution containing metal with a solution inan immiscible solvent of a composition according to the presentinvention;

b. separating the aqueous and solvent phases, the latter containingmetal complex;

c. contacting the solvent phase with an aqueous mineral acid; and

d. separating the solvent phase from the aqueous phase containing metalin the form of a salt of the mineral acid.

Preferably the metal is copper or nickel, more preferably copper itself.

o-hydroxyaryloximes generally of value for extracting metal values fromaqueous solutions of metal salts are well known and include for examplealkyl or alkoxysalicylaldoximes as described in Belgian Patent Nos.796,835, substituted, e.g. by alkyl or alkoxy groups, o-hydroxyarylalkyl ketoximes as described in British specification 1,322,532, GermanOffenlegungsschrift 2407200 and Belgian Patent No. 804,031,o-hydroxyarylbenzyl ketoximes as described in Belgian Patent No.804,030, and o-hydroxybenzophenone oximes as described in U.S. Pat. Nos.3,428,449 and 3,655,347. In order to confer adequate solubility of theoxime and its metal derivative in the organic solvents the oximes shouldcontain groups, e.g. alkyl, alkylene or cycloalkyl groups containing atleast three carbon atoms and preferably not more than 20 carbon atoms.The solubility is generally further enhanced by the use of mixtures ofoximes. Preferred compounds contain C7-Cl5 alkyl groups.

Of the above o-hydroxyaryloximes only those which are strong metalextractants are useful in the process of the invention. Theseo-hydroxyaryloximes are defined as those which in 0.2 molar solution inan aliphatic hydrocarbon solution when loaded with 50% of thetheoretical uptake of copper will be in equilibrium with a 0.1 molarsolution of copper as copper perchlorate at a pH less than 1. Incontrast o-hydroxyaryl ketoximes which are devoid of electronwithdrawing substituents in the 3-position, examples which are describedin British Specification 1,322,532, U.S. Pat. No. 3,428,449 and BelgianPatents Nos. 804030 and 804031, in the above test are usually inequilibrium at pH about 1.2 or higher and are not suitable for use inthe present invention on their own, but they may be used in admixturewith the compositions of the present invention.

Alkylphenols, as described in our UK Patent No 1,549,615 may also bepresent in amounts from 10 to 300% by weight of the oxime.

Particularly useful, owing to their ability to deal with aqueoussolutions containing high copper concentrations and their rapid rates ofmetal transfer, are the alkyl salicylaldoximes especially wherein thealkyl groups are branched chain alkyl groups containing at least fivecarbon atoms and mixtures of these, for example 4-nonyl-salicylaldoximesand mixed 5-heptylsalicylaldoximes, and particularly mixtures of2-hydroxy 5-nonylbenzaldoximes, in which the components of the mixturediffer in configuration of the branched chain nonyl group, derived byformylation and oximation from the mixed p-nonylphenols obtained bycondensation of phenol with propylene trimer, and mixtures of2-hydroxy-5-heptylbenzaldoximes, in which the components of the mixturediffer in configuration of the heptyl group, similarly derived fromphenol-heptylene condensate.

Also useful however are mixtures of strong o-hydroxy benzaldoximes ofthe type described above, and the weaker o-hydroxyarylketoximes of thetype referred to earlier and described in U.S. Pat. No 3,428,449 andBelgian Patents Nos. 804030 and 804031. Such mixtures are described inEuropean Publication No. 85522. The performance of these mixtures canalso be suitably modified by the incorporation of the highly branchedaliphatic or aliphatic-aromatic alcohols or esters of this invention.

As alcohols in these compositions and processes may be used eithersaturated or unsaturated aliphatic hydrocarbon alcohols or polyolscontaining 14 to 30, preferably 15 to 25 carbon atoms. The alcohols arepreferably highly branched with the hydroxyl group being locatedapproximately midway along the hydrocarbon backbone. Especiallypreferred for this application are the branched chain alcohols that maybe made by condensation of short alcohols by the Guerbet process. Suchalcohols are sometimes referred to as Guerbet alcohols. Optionally thealcohols may contain an aromatic group or other functional group,particularly an ester group.

Especially useful in these compositions are alcohols synthesised fromhighly branched precursors leading to very highly branched Guerbetalcohols containing a large number of terminal methyl groups.

It has been found that a particularly efficient modifier is the highlybranched isohexadecyl alcohol or isooctadecyl alcohol the latter being2-(1,3,3-trimethylbutyl) -5,7,7 trimethyl octanol.

As esters in these compositions and processes may be used eithersaturated or unsaturated aliphatic or aromatic-aliphatic esterscontaining from 10 to 30 carbon atoms. The esters may be polyesters,especially diesters. The esters are preferably highly branched.Optionally the esters may contain other functional group, moreparticularly a hydroxyl group.

In the context of this invention `highly branched' means that the ratioof the number of methyl carbons to non methyl carbons is higher than1:5.

Especially useful in these compositions and processes are esters derivedfrom certain diacids, preferably branched diacids. As examples may bequoted 2,2,4-trimethyl-1,3-pentanediol diisobutyrate and the benzoicacid ester of 2,2,4-trimethyl-l,3-pentanediol monoisobutyrate. Thelatter esters are commercialy available.

Mixture of esters or alcohols with other modifiers or with other estersor alcohols according to the present invention may also be usedadvantageously.

The amount of oxime used will depend upon the concentration of metalsalt in the aqueous solution and the plant design. It is preferredhowever to use from 5 g to 300 g of oxime per liter of organic solution.Higher concentrations afford organic phases of too high viscosity forconvenient handling and lower concentrations involve the use ofunnecessarily large volumes of solvent.

For use with aqueous solutions containing 1 g or more per liter of metalsuch as copper it is preferred to use 20 to 200 g of oxime per liter oforganic solution in conjunction with an amount of alcohol or estersuitably from 10% to 200% of the weight of the oxime, and especiallyfrom 20% to 100%. The effect of the alcohol or ester is more marked thehigher the concentration of oxime and comparatively lower proportions ofthe modifier with respect to the oxime are required to bring about asatisfactory improvement in strip efficiency when operating at highconcentrations.

The first and second steps of the process may conveniently be carriedout by bringing together the aqueous solution and the solution of theoxime in the organic solvent at a suitable temperature, usually ambienttemperature, although somewhat higher temperatures may be used ifoperationally convenient, agitating or otherwise disturbing the mixtureof liquids so that the area of the water-solvent interfacial layer isincreased in order to promote complex formation and extraction, and thendecreasing the agitation or disturbance so that the aqueous and solventlayers settle and can be conveniently separated. The process may becarried out in a batchwise manner or preferably continuously.

The amount of organic solvent to be used may be chosen to suit thevolume of aqueous solution to be extracted, the concentration of metals,and the plant available to carry out the process. It is preferred,especially when operating the process continuously, to bring togetherapproximately equal volumes of the organic solution and the aqueoussolution.

The conditions, particularly pH values, under which first and secondsteps of the process are carried out are chosen to suit the metal ormetals present in the aqueous solution. It is-generally desirable thatunder the chosen conditions any other metals present should not formstable complex compounds with the oxime in order that substantially onlythe desired metal is extracted from the aqueous solution. Sinceformation of the complex compound may involve the liberation of acid, itmay be necessary to add e.g. alkali during the process to maintain thepH within the desired range in which the metal complex is stable but itis generally preferable to avoid this, especially in acontinuously-operated process. The process of the invention isespecially suitable for use with copper since the metal forms a complexwith o-hydroxyaryloximes which is stable at low pH values and byoperating at pH below 3 copper can be extracted substantially free fromiron, cobalt and nickel. As organic solvents there may be used anymobile organic solvent mixture of solvents which is immiscible withwater and, under the pH conditions used, inert to water, and to theoxime, especially aliphatic, alicyclic and aromatic hydrocarbons andmixtures of any of these, particularly mixtures which have little or noaromatic hydrocarbon component, and halogenated particularly chlorinatedhydrocarbons including, as solvents more dense than water, highlyhalogenated hydrocarbons such as perchloroethylene, trichloroethane,trichloroethylene and chloroform.

The third and fourth steps of the process may conveniently be carriedout by bringing together the metal-bearing solution of the oxime in theorganic solvent, obtained from the second stage of the process, and anaqueous solution of a mineral acid at a suitable temperature, usuallyambient temperature, although somewhat higher temperatures may be usedif operationally convenient, agitating or otherwise disturbing themixture of liquids so that the area of the aqueous-solvent interfaciallayer is increased in order to promote decomposition of the complex andrecovery of the metal and then decreasing the agitation or disturbanceso that the aqueous and solvent layer settle and then separating thelayers. Suitable relative volumes of organic to aqueous phases are thoseconventionally used in metal extraction processes for example 1:1. Inthe stripping stage, such value will be typically 5:1. The process maybe carried out in a batchwise manner or preferably continuously. Thestripped organic layer, containing regenerated oxime, the modifier andsome residual copper may be re-used in the first step of the process.The aqueous layer containing metal salt may be treated in anyconventional manner, especially by electrolysis, to provide the metal.

The stripping acid is preferably sulphuric, suitable strengths beingfrom 100 to 250g. per liter. After removal of a convenient part of themetal by electrolysis the recovered aqueous acid, containing residualmetal salt, may be re-used in the third step of the process.

The invention is illustrated by the following non-limitative examples:

EXAMPLE 1

50 parts of a solution containing 50 g per liter of 2-hydroxy-5-nonylbenzaldoxime in Escaid 100 (an aliphatic Kerosenetype solvent) wasstirred vigorously at 25 deg. Celsius with 100 parts of an aqueoussolution containing 3.0 g per liter of copper as the sulphate at aninitial pH of 2.0. After 15 minutes the stirring was stopped, the phasesallowed to settle and a portion of the solvent phase removed andanalysed for copper.

25 parts of the copper loaded organic phase were then stirred vigorouslyat 25 deg. Celsius for 15 minutes with 50 parts of an aqueous strippingsolution containing 30 g per liter copper as the suplate and 150 g perliter of sulphuric acid. Again, the phases were allowed to settle andthe organic phase analysed for copper.

The results showed that the organic phase following extraction contained5.29 g per liter of copper and after stripping contained 3.35 g perliter of copper, indicating a recovery of copper of 1.94 g per liter ofextractant solution used.

To demonstrate the improvement obtained by the incorporation of a longchain alcohol in the extractant composition the above experiment wasrepeated using an Escaid 100 solution containing 50 g per liter of2-hydroxy-5-nonyl benzaldoxime and 25 g per liter of a highly branchediso-octadecyl alcohol (ex. Hoechst of Germany). The copper content ofthe organic phase after extraction was 5.01 g per liter and afterstripping it was 2.29 g per liter, indicating a copper recovery of 2.72per liter of extractant solution used, this being an increase of 40%over the recovery in the absence of the iso-octadecyl alcohol.

EXAMPLE 2

A pilot trial was carried out using a real mine feed solution which wasfed to a small solvent extraction plant comprising two extraction mixersettlers and two strip mixer settlers arranged in series with countercurrent flows of organic and aqueous phases through the extraction andstripping stages. The aqueous feed solution from a dump leachingoperation contained a variable concentration of metals during theseveral week period of the trial, the concentration of copper beingbetween 2.0 and 4.5 g per liter, of iron being from 22 to 30 g per literat a pH of 1.6. Stripping was carried out using an aqueous solutioncontaining 30 g per liter of copper and 165 g per liter of sulphuricacid. The organic phase comprised an 8% by volume solution of acomposition containing 50% by weight 2-hydroxy-5-nonyl benzaldoxime, 25%by weight iso-octadecyl alcohol, the balance being Escaid 100. Theorganic diluent was Kermac 470 B, another type of high flash pointKerosene commonly used as diluent in solvent extraction processes.

The organic:aqueous phase ratio in extraction stages was between 1.0 and1.25 and in stripping was 6.3 to 6.6 overall, but by recycling theaqueous phase was arranged to be approximately 1.0 in the mixer. In bothextraction stages and the first stripping stage the organic extractantsolution formed the continuous phase, but in the second strip stage theaqueous solution formed the continuous phase during mixing. The meantemperature during the course of the trial was 23 deg. Celsius.

Samples were taken each day of all streams exiting the extraction andstrip stages and measurements made of the level of one phase entrainedin the other. It is extremely important to maintain low levels ofentrainment because entrainment of organic in the aqueous phase reads toloss of the extractant from the system, and entrainment of aqueous feedin the organic stream proceeding to stripping represents a means bywhich undesired metals such as iron can be carried over to theelectrolyte from which copper is subsequently removed by electrowinning.

The entrainment levels, based on the average of 14 or more measurementsfor each stream were as follows: All figures are parts per million byvolume.

Aqueous in organic stream from 1st extractant stage 98 ppm

Aqueous in organic stream from 2nd strip stage 26 ppm

Organic in aqueous stream from 2nd extraction stage 40 ppm

Organic in aqueous stream from 1st strip stage l4 ppm

Entrainment levels of aqueous in the organic stream exiting the firstextraction stage of greater than 1000 ppm have often been recorded whenoperating with an extractant solution comprising an 8% solution inKerosene (Kermac 470 B) of a composition containing 2-hydroxy-5-nonylbenzaldoxime in admixture with 5-nonyl phenol in approximately 1:1 ratioby weight.

There is clearly an advantage for the composition of the inventiondescribed here as the organic system exiting the first extraction stageis passed to the stripping circuit and represents a means of transfer ofunwanted iron the copper electrolyte.

During the course of this trial measurements were also made of the depthof interfacial sludge that accumulated, in particular in the firstextraction stage. Using the composition of this invention described inthis example, the depth of this material was found to approximatelyl0cm. In a similar trial using an extractant solution based on aformulation containing 2-hydroxy-5-dodecyl benzaldoxime and tridecylalcohol in approximately 2:5:1 ratio by weight, the amount of sludgemeasured at the same point in the settler of first extraction stage wasfound to extend to a depth of 25cm.

EXAMPLE 3

In order to compare the copper over iron selectivity of a composition ofthis invention with the selectivity of a known composition containingtridecyl alcohol, the following experiment was carried out.

Solutions in Escaid 100 were prepared containing 9% by volume of twodifferent extractant compositions. The first extractant composition wasa formulation containing 50% by weight of 2-hydroxy-5-nonylbenzaldoxime, 25% by weight of iso-octadecyl alcohol, the balance beingEscaid 100. The second extractant composition was a formulationcontaining 50% by weight of 2-hydroxy-5-nonyl benzaldoxime, 25% byweight of tridecyl alcohol, the balance being Escaid 100.

A portion of each extractant solution was stirred vigorously at 25 deg.Celsius with an equal volume of an aqueous solution containing 3.0 g perliter copper and 3.0 g per liter ferric iron as their sulphates at a pHof 2.0. The dispersions were sampled after 2 minutes and again after 15minutes stirring, 2-3 minutes being a typical mean residence time in themixer of commercial operations, and 15 minutes being long enough toestablish full equilibrium.

The organic and aqueous phases were separated and the organic phaseanalysed for iron by atomic absorption spectrophotometry. The amount ofiron co-extracted with the copper after 2 minutes was found to be 55 mgper liter in the case of the composition containing tridecyl alcohol butonly l0mg per liter for a composition of this invention, in this casecontaining iso-octadecyl alcohol. After 15 minutes stirring the level ofiron in the solution containing the tridecyl alcohol modified reagentwas 88 mg per liter while that in the solution containing oxime modifiedwith iso octadecyl alcohol was again only 10 mg per liter.

EXAMPLE 4

A number of laboratory scale mixer settler units were set up, eachconsisting of a mixer box of volume 560 cm3 from which theaqueous/organic dispersion overflowed into a settler compartiment havinga depth of 35 cm. Organic and aqueous streams were fed in at the base ofthe mixer and agitated by a six vaned impeller of diameter 25 mm whichwas rotated at 1100 revolutions per minute. A baffle was employed suchthat dispersion overflowing from the mixer was introduced at a pointmidway between the top and the bottom of the settler. Another baffle wasplaced partway along the length of the settler to provide an effectivesettler volume equivalent to a flow of 61 liters per square meter perminute.

A number of extractants formulations were made up which each contained50 parts by weight of 2-hydroxy-5-nonyl-benzaldoxime in addition tomodifiers as follows: Extractant A 25 parts isooctadecyl alcohol

Extractant B 25 parts isohexadecyl alcohol

Extractant C 30 parts 2-octyl dodecanol

Extractant D 17 parts tridecanol

In each case the composition was made up to 100 parts by weight by theaddition of Escaid 100 (high flash point kerosene). The amount ofmodifier used in extractant compositions B, C and D was chosen so as togive a reagent with the same extractive strength and strippingcharacteristics as extractant A. Solutions were prepared containing 10%by volume of each of these extractant compositions in Chevron IonExchange Solvent (a high flash point Kerosene used as carrier in solventextraction processes). A different reagent solution was pumped to eachmixer settler at a rate of 45 ml/min together with 45 ml of a minesolution. The mine solution used was common to all four mixer settlersand was obtained from a leaching operation carried out at a mine site inArizona, USA. It contained approximately 2 g/ liter copper at pH 2. Themixer settlers were run continuously in runs lasting at least six hours.The organic phases were collected in receivers from which they wererecycled to the appropriate mixer settler. The spent aqueous solutionswere run to drain after one passage through the mixer settlers. Duringthe course of these runs, a number of samples were taken of both theorganic phase and the aqueous phase near to the point at which thesephases exited to settler. These samples were then centrifuged inspecially designed graduated vessels to determine the levels ofentrainment of both organic in aqueous and aqueous in organic. Twoseries of experiments were carried out. In the first; steps were takenat start up to ensure that the organic reagent solutions formed thecontinuous phase in the mixers. In the second series, the aqueous feedformed the continuous phase. The entrainment levels obtained, based onthe means of several measurements are tabulated below (O=organic,A=aqueous).

    ______________________________________                                                       Entrainment                                                                   level in ppm by volume                                         Ex-                  For organic                                              tractant             continuous                                               Com-                 dispersion  For aqueous                                  position                                                                             Modifier      O in A  A in O                                                                              O in A                                                                              A in O                               ______________________________________                                        A      isooctadecyl alcohol                                                                        154      865   45   194                                  B      isohexadecyl alcohol                                                                        196      800   27   183                                  C      2-octyl dodecanol                                                                           280     1175  153   1813                                 D      tridecanol    204     1250  190   400                                  ______________________________________                                    

It is clearly evident from the data that considerably lower levels ofentrainment are obtained where the modifier is a highly branched Cl6 orCl8 alcohol, compared to the slightly branched C20 alcohol and thepartially branched but lower molecular weight C 13 alcohol. Particularlylow level of aqueous in organic entrainment are obtained withformulations A and B when the mixer is operated with aqueous forming thecontinuous phase, thus minimising physical transfer of unwanted metalssuch as iron present in the aqueous feed solution. It is also noted thatparticularly low levels of entrainment of organic in the aqueous areobtaiend with formulations A and B, employing the highly branchedalcohol as modifier. This is a considerable benefit because entrainmentof organic reagent solution in the spent aqueous phase, which issubsequently discarded, represents a major source of loss of reagentfrom the system and is one of the major running costs in a solventextraction operation.

EXAMPLE 5

A number of experiments were carried out using mixer settlers asdescribed in Example 4. The reagent solutions used were 10% solutions byvolume of various formulations made up in Chevron Ion Exchange Solvent.Each of the formulations used contained 50 parts by weight of2-hydroxy-5-nonyl benzaldoxime, a modifier as tabulated below plusEscaid 100 to make up to 100 parts by weight. The modifiers used in thevarious compositions were as follows

Composition A contained 25 parts by weight of isooctadecyl alcohol

Composition B contained 30 parts by weight of 2-octyl dodecyl alcohol

Composition C contained 30 parts by weight of a highly branched ester offormula 2,2,4-trimethyl-l,3-pentane diol diisobutyrate Composition Dcontained 40 parts by weight of methyl laurate, a straight chain fattyester

The amount of modifier incorporated into each of the compositions B, Cand D was such as to give a composition with the same extractivestrength for copper as composition A. Each of these reagent solutionswas pumped to a different mixer settler at a rate of approximately 45 mlper minute. The aqueous feed fed to each mixer settler was a leachsolution from a mine known to produce severe crud problems in itssolvent extraction plant. This particular mine feed solution containedapproximately 3.0 g per liter of copper and 30 g per liter ferric ion ata pH of 2. The aqueous feed was pumped to each mixer settler and a rateof crud generation ascertained over the course of each experiment.

The mixer settlers were run continuously in runs lasting at least sixhours. The organic phases were collected in receivers from which theywere recycled to the appropriate mixer settler. The spent aqueoussolutions were run to drain after one passage through the mixersettlers. During the course of the runs a number of samples were takenof both the organic and the aqueous phase near to the point at whichthese phases exited the settler. These samples were then centrifuged inespecially designed graduated vessels to determine the levels ofentrainment of both organic in aqueous and aqueous in organic.

Two series of experiments were carried out. In the first steps weretaken at start up to ensure that the organic reagent solutions formedthe continuous phase in the mixers. In the second series, the aqueousfeed formed the continuous phase. The entrainmenet levels obtained,based on the means of several measurements are tabulated below(O=organic, A=aqueous) Levels of entrainment (ppm by volume) and rate ofcrud generation (mm/hr) for various reagent compositions.

    ______________________________________                                                  For organic continuous dispersion                                   Extractant      Entrainment (ppm)                                                                           Rate of crud                                    composition                                                                             O in A      A in O  generation                                      ______________________________________                                        A          87         361     1.96                                            B         313         792     3.4                                             C          53          25     0.57                                            D         300         1125    13.6                                            ______________________________________                                                  For aqueous continuous dispersion                                   Extractant      Entrainment (ppm)                                                                           Rate of crud                                    composition                                                                             O in A      A in O  generation                                      ______________________________________                                        A         107         232     0.93                                            B          50         350     5                                               C          94          50     1.0                                             D         360         3000    11.5                                            ______________________________________                                    

These data demonstrate that in the case of both the alcohol modifiersand the ester modifiers the highly branched compounds are generallysuperior as modifiers with respect to the levels of entrainmentproduced. It is quite evident that the rate of build up of undesirablecrud is considerably less in the case of formulations A and C employinghigly branched modifiers than in the case of B and D where the modifiercontain linear alkyl chains. In addition it is noted that estercomposition C is generally superior to the alcohol composition B.

EXAMPLE 6

100 parts of a solution containing 50 g per liter of 2-hydroxy-5-nonylbenzaldoxime in Escaid 100 (an 80% aliphatic Kerosene type solvent) wasloaded to its maximum loading capacity of copper by stirring vigorouslyat 25 deg C for 5 minutes with a solution containing 10 g per liter ofcopper, buffered at pH 4.5 by the addition of 54 g per liter of sodiumacetate. The phases were allowed to separate and the organic phase foundby analysis to contain 6.0 g per liter of copper. 50 parts of the copperloaded extractant solution were shaken vigorously at 25 deg C for 2minutes with 50 parts of an aqueous strip solution simulating a spentelectrolyte recovery solution. This contained 30 g per liter of copper,added as the sulphate, and 150 g per liter of sulphuric acid. Aftershaking, the phases were allowed to settle, the aqueous solution run offand replaced by a fresh 50 parts of strip solution. Shaking wascontinued as before and the above procedure repeated until the organicphase had been contacted with four separate portions of strip acid. Aportion of the organic phase was then removed for analysis and found tocontain 3.3 g per liter of copper. The above experiment was repeatedusing an extractant solution containing 50 g per liter of2-hydroxy-5-nonylbenzaldoxime and 50 g per liter of2,2,4-trimethyl-l,3-pentane diol diisobutyrate. It was found that afterfour contacts with strip acid the copper loading in the organic phasehad been reduced to 1.47 g per liter, a marked improvement in stripping.

EXAMPLE 7

A solution (I) was prepared containing 25 g per liter of2-hydroxy-5-nonylbenzaldoxime and 25 g per liter of 4-nonyl phenol inEscaid 100. Such a composition is commonly used in commercial operationsfor the solvent extraction of copper. An aqueous solution (II) wasprepared containing 3.0 g per liter of copper and 30 g per liter offerric iron as their respective sulphates, with a small amount ofsulphuric acid to give a pH of 2.0. 80 parts of the extractant solution(I) were equilibrated by stirring vigorously for 15 minutes at 25 deg Cwith 80 parts of aqueous solution (II) after which the solutions wereseparated and a portion of the metal bearing organic solution (I) setaside for analysis. 40 parts of this part-loaded organic solution weregiven a second contact with aqueous feed by stirring for 15 minutes withanother 40 parts of solution II. Again the phases were separated and aportion of the organic solution set aside for analysis. In anotherexperiment an extractant solution (III) was prepared containing 25 g perliter of 2-hydroxy-5nonyl benzaldoxime and 15 g per liter of2,2,4-trimethyl-l,3-(pentane diol diisobutyrate. This ratio of oxime tomodifier was chosen as it gives the same copper transfer properties(strip performance) as solution (I). The above experiment was repeated,portion of solutions (III) being equilibrated twice with fresh portionsof solution (II). Again portions of the organic phase were set aside foranalysis after each contact. The various samples of organic solutionswere filtered twice through Whatman filter paper to removed anyentrained aqueous and then analysed to determine the amount of copperand iron present in each. Iron was determined directly by atomicabsorption spectrophotometry of the organic solutions. Copper wasdetermined by stripping it into aqueous solution by several contactswith 300 g per liter sulphuric acid followed by neutralisation, additionof potassium iodide and titration of the liberated iodine with standardthiosulphate solution. The analytical results are tabulated belowtogether with results obtained using as modifiers tridecanol (IV),isooctadecyl alcohol (V), a mixture of 2 hydroxy-5-dodecyl benzaldoximeand 2 hydroxy-5-nonyl benzophenone oxime available commercially underthe trade name Lix 864 (VI) and a mixture of 2 hydroxy-5-dodecylbenzaldoxime and 2-hydroxy-5-nonyl acetophenone oxime (VII) availableunder the trade name Lix 984.

    ______________________________________                                               Analysis of organic phase                                                     1st contact        2nd contact                                         Reagent  Cu      Fe      Cu/Fe  Cu   Fe    Cu/Fe                              composition                                                                            g/l     mg/l    ratio  g/l  mg/l  ratio                              ______________________________________                                        I        2.78    54       52    3.01 7.77  386                                III      2.756   13.0    213    3.02 2.5   1210                               IV       2.783   19.1    147    3.02 4.1   736                                V        2.80    16.0    175    3.02 3.1   976                                VI       2.77    120      23    3.024                                                                              92     33                                VII      2.726   48       57    3.011                                                                              25    120                                ______________________________________                                    

This clearly demonstrate that by replacement of the nonyl phenolmodifier with 2,2,4-trimethyl-l,3-pentane diol diisobutyrate , aconsiderable improvement is obtained in the ratio of copper to ironextracted. Similarly a formulation containing the alcohol modifier V inaccordance with this invention provides better selectivity than priorart tridecanol or mixtures of aldoxime and ketoxime.

We claim:
 1. A composition for use in extracting copper values fromaqueous solutions of metal salts which comprises(A) one or moreo-hydroxyaryloximes containing at least 5 aliphatic or alicyclic carbonatoms which are strong metal extractants which, in 0.2 molar solution inan aliphatic hydrocarbon solution when loaded with 50% of thetheroretical uptake of copper, will be in equilibrium with 0.1 molarsolution of copper as copper perchlorate at a pH less than 1; and (B)one or more branched chain aliphatic or aromatic-aliphatic alcoholscontaining 14 to 30 carbon atoms or aliphatic or aliphatic-aromaticesters containing 10 to 30 carbon atoms, wherein the ratio of the numberof methyl carbon atoms to the number of non-methyl carbon atoms ishigher than 1:5, and which is selected from the group consisting ofhighly branched isohexadecyl alcohol, highly branched isooctadecylalcohol and a diester of 2,2,4,-trimethyl-1,3pentanediol; and whereinthe weight ratio of A to B is in the range 10:1 to 1:3.
 2. Thecomposition of claim 1 in which component A is an oxime which containsfrom 7 to 15 aliphatic or alicyclic carbon atoms.
 3. The composition ofclaim 1 which is in solution in a water immiscible organic solvent. 4.The composition of claim 1 in which component B is an alcohol or esterwherein the ratio of the number of methyl carbon atoms to the number ofnon-methyl carbon atoms is higher than 1:3.
 5. The composition of claim1 wherein component B is 2-(1,3,3-trimethyl butyl)-5,7,7-trimethyloctanol.
 6. The composition of claim 1 wherein component B is a diesterwhich contains 14 to 25 carbon atoms.
 7. The composition of claim 1wherein component B is 2,2,4-trimethyl-1,3-pentanediol diisobutyrate. 8.The composition of claim 1 in which the branched chain ester is thebenzoic acid ester of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate.9. A composition for use in extracting copper values from aqueoussolutions of metal salts which comprises(A) one or moreo-hydroxyaryloximes containing at least 5 aliphatic or alicyclic carbonatoms which are strong metal extractants which, in 0.2 molar solution inan aliphatic hydrocarbon solution when loaded with 50% of thetheoretical uptake of copper, will be in equilibrium with 0.1 molarsolution of copper as copper perchlorate at a pH less than 1; and (B)one or more branched chain aliphatic or aliphatic-aromatic esterscontaining 10 to 30 carbon atoms, wherein the ratio of the number ofmethyl carbon atoms to the number of non-methyl carbon atoms is higherthan 1:5, and which is a diester of 2,2,4-trimethyl-1,3-pentanediol; andwherein the weight ratio of A to B is in the range 10:1 to 1:3.